Hydrogen (H2) gas is highly volatile and, when in contact with oxygen, can become extremely flammable and highly explosive. The use of effective hydrogen sensors to accurately and quickly respond to hydrogen gas leaks and to monitor manufacturing and distribution will be crucial for the safe deployment of all hydrogen-based applications. Hydrogen sensors must be sensitive enough to discriminate between ambient low-level traces of hydrogen and those that are generated by a hydrogen leak.
Palladium based hydrogen sensors have a unique advantage in that the surface of palladium can act catalytically to break the H—H bond in diatomic hydrogen, allowing monatomic hydrogen to diffuse into the material. Furthermore, palladium can dissolve more than 600 times its own volume of hydrogen, but dissolves little of the other common gases such as nitrogen, oxygen, nitrogen monoxide, carbon dioxide, and carbon monoxide. This allows palladium to be the most selective hydrogen sensing material. Finally, the palladium hydrogenation process is reversible at room temperature, enabling simpler designs.
In the presence of hydrogen the resistance of palladium will change due to the formation of a solid solution of (at low H2 pressure, α-phase) or a hydride (at high H2 pressure, β-phase). The level of dissolved hydrogen changes the electrical resistivity of the metal and also its volume due to the formation of metal hydride. Consequently, conventional hydrogen sensors have been fabricated from bulk materials, by applying palladium metal to bulk substrates. Recent developments in semiconductor processes have also allowed the detection and measurement of hydrogen using Schottky junctions including palladium; in this case, the metal-semiconductor junctions are affected by the diffusion of hydrogen. All these sensors use a change in resistivity of the palladium caused by the change in crystal structure to detect hydrogen, measured by injection of current.
Typically, technology which relies upon the resistive response of hydrogen diffusion into palladium cannot measure hydrogen concentrations above 4% (volume/volume; the threshold of hydrogen in air which can cause an explosion). Furthermore, failure of the sensors occurs upon exposure of the device to hydrogen concentration greater than 5% (volume/volume). The major failure modes of the sensors seem to be some form of stiction (static friction), caused by the injection of direct current through the bulk material, thus altering the crystal structure.